Organic light emitting diode (OLED) devices are attractive because of their low driving voltage, high luminance, wide-angle viewing, and capability for full-color flat emission displays. Tang et al. described a multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
Efficient white light producing OLED devices are considered low cost alternatives for several applications such as paper-thin light sources, backlights in LCD displays, automotive dome lights, and office lighting. White light producing OLED devices should be bright, efficient, and generally have a broad emission spectrum that covers most of the visible wavelength range. Herein the term “white” or “substantially white” will be used broadly to mean light that is perceived as white or off-white.
The following patents and publications disclose the preparation of organic OLED devices capable of emitting white light, comprising a hole-transporting layer and an organic luminescent layer interposed between a pair of electrodes.
White light producing OLED devices have been reported before by J. Shi in U.S. Pat. No. 5,683,823 wherein the luminescent layer includes red and blue light emitting materials uniformly dispersed in a host emitting material. This device has good electroluminescent characteristics, but the concentrations of the red and blue dopants are very small, such as 0.12% and 0.25% of the host material. These concentrations are difficult to control during large-scale manufacturing. Sato et al. in JP 07,142,169 discloses an OLED device capable of emitting white light, made by forming a blue light emitting layer next to the hole-transporting layer and followed by a green light emitting layer having a region containing a red fluorescent layer.
Kido et al., in Science, Vol. 267, p. 1332 (1995) and in Applied Physics Letters, Vol. 64, p. 815 (1994), report a white light producing OLED device. In this device, three emitter layers with different carrier transport properties, each emitting blue, green, or red light, are used to generate white light. Littman et al. in U.S. Pat. No. 5,405,709 disclose another white emitting device, which is capable of emitting white light in response to hole-electron recombination, and comprises a fluorescent in a visible light range from bluish green to red. Deshpande et al., in Applied Physics Letters, Vol. 75, p. 888 (1999), describe a white OLED device using red, blue, and green luminescent layers separated by a hole blocking layer.
OLED devices typically have at least one transparent electrode, which is often fabricated from a conductive oxide such as indium-tin oxide (ITO). Such materials have sufficient conductivity for displays, in which the individual pixels are on the order of 1 mm or less. However, the conductivity of such transparent electrodes can be insufficient for applications with much larger individual emitting units, such as panel lighting. This disadvantage can be overcome by making the emitting elements in narrow stripes, but registration difficulties would make such a device more difficult to fabricate, increasing the manufacturing cost. Furthermore, such transparent electrodes are themselves costly and add to the manufacturing cost.